Glucagon is produced by the pancreatic A-cell and released in response to low blood glucose levels. Its major site of action is the liver where it stimulates glucose production. It is thus the major hormone counteracting insulin in blood glucose homeostasis (Unger, R. H. and L. Orci (1990). Glucagon, in: Diabetes Mellitus, 4th ed. New York, Elsevier. pp 104-120).
Glucagon is processed from a larger precursor by limited proteolysis. Molecular cloning of the glucagon gene revealed that the proglucagon precursor contained not only glucagon but also two additional glucagon-like peptides named GLP-1 and GLP-2. GLP-1 and GLP-2 are encoded by separate exons suggesting distinct biological activities. It was later demonstrated that the proglucagon precursor was subjected to differential processing in the three different tissues known to produce proglucagon: the pancreatic A-cell, the intestinal L-cell, and in the central nervous system (CNS). Glucagon is thus selectively excised from the precursor in the islet A-cell, while GLP-1 and GLP-2 are selectively liberated from the intestinal L-cell and the CNS reviewed in (Unger, R. H. and L. Orci (1990). Glucagon. in: Diabetes Mellitus, 4th ed. New York, Elsevier. pp 104-120)!.
Specific GLP-1 receptors have been identified (Thorens, B. (1992) Proc. Natl. Acad. Sci. USA 89: 8641-8645) which are clearly distinct from the glucagon receptor (L. J Jelinek, et al. (1993) Science 259: 1614-1616) and they have different tissue distributions (R. V Campos, et al. (1994) Endocrinology 134: 2156-2164). GLP-1 is released from the L-cell after a meal and functions as an incretin hormone (i.e. it potentiates glucose induced insulin release from the pancreatic B-cell). The GLP-1 receptor is thus expressed at high levels on the surface of islet B-cells (K. Moens, et al. (1996) Diabetes 45: 257-261).
Induction of intestinal epithelial proliferation by GLP-2 was demonstrated (Drucker, D. J. et al (1996) Proc. Natl. Acad. Sci. USA 93: 7911-7916) and treatment of gastrointestinal deseases by cells grown in GLP-2 medium was disclosed (Drucker, D. J and Keneford, J. R., WO 96/32414).
No GLP-2 receptor has so far been reported.
Proglucagon derived peptides and feeding behaviour
We have previously reported the derivation and establishment of transplantable anorectic glucagonomas (O. D. Madsen et al. (1993) Endocrinology 133: 2022-2030) as well as of hypoglycemic insulinomas in the rat (O. D Madsen, et al. (1988) Proc. NatI. Acad. Sci. USA 85: 6652-6656). Such tumors can be derived from common clonal origin of pluripotent MSL-cells (O. D. Madsen, et al. (1986) J. Cell Biol. 103: 2025-2034) and reflects a maturation process towards islet A-cell and B-cells, respectively (O. D. Madsen, et al. (1993) Endocrinology 133: 2022-2030).
The glucagonoma associated anorexia is very severe: it has an acute onset and leads after few days to a complete stop in food intake. This severity of anorexia is hardly matched by other experimental tumors in rodents and suggests the production by the glucagonoma of a very powerful satiety factor which acts by a peripheral route of administration. It has previously been demonstrated that the anorectic glucagonomas displayed an unphysiological processing resulting in the formation of both glucagon and GLP-1 (O. D. Madsen, et al. (1993) Endocrinology 133: 2022-2030). Moreover, a non-anorectic glucagonoma variant was unable to process the precursor (O. D. Madsen, et al. (1995) Scand. J. Clin. Lab. Invest. 55, suppl 220: 27-36. Weight loss is mentioned as a component also of the glucagonoma syndrome in man (J. J. Holst (1985) Glucagon-producing tumors, in: Hormone-producing tumors of the gastrointestinal tract. New York, Churchill Livingstone. pp 57-84) although with a high degree of variability among different patients (S. J. Bhathena, et al. (1981). Glucagonoma and glucagonoma syndrome, in: Glucagon. Physiology, pathophysiology and morphology of the pancreatic A-cells. New York, Elsevier. 413-438).
Glucagon
Glucagon has been shown to be involved in the regulation of spontaneous meal size in rats but the overall effect is minimal and is exerted via the vagal connections to the liver (N. Geary, et al. (1993) Am. J. Physiol. 264: R116-R122). This effect is observed only by hepatic portal infusion of glucagon while intraperitoneal administration of pharmacological doses show no effect on food intake in fasted rats (O. D. Madsen, et al. (1993) Endocrinology 133: 2022-2030).
GLP-1
A central role for GLP-1 in the regulation of feeding was recently reported (M. D. Turton, et al. (1996) Nature 379: 69-72). Intracerebroventricular (ICV) administration of GLP-1 inhibited feeding in fasted rats. Again peripheral administration of GLP-1 had no effect on feeding behaviour (M. D. Turton, et al. (1996) Nature 379: 69-72; O. D. Madsen, et al. (1993) Endocrinology 133: 2022-2030) suggesting that tumor produced GLP-1 may not contribute significantly to the observed anorexia.